The generation of superoxide and hydrogen peroxide by exposure of fluorescent whitening agents to UVA radiation and its relevance to the rapid photoyellowing of whitened wool

Various fluorescent whitening agents (FWAs), chosen from the three major classes used commercially on textiles (stilbenes, pyrazolines and coumarins), produced hydrogen peroxide and superoxide radical anions when irradiated in aqueous solution with UVA light at 366 nm, near their absorption maximum. In contrast, none of these FWAs produced singlet oxygen on irradiation under similar aqueous conditions. The formation of superoxide, rather than 1O2, suggests a mechanism where the excited singlet state of the FWA may undergo ionization to produce either an FWA radical cation and a free electron which is accepted by molecular oxygen, or an electron transfer reaction via formation of semi-reduced and semi-oxidized FWA radicals. Aqueous tryptophan also generates hydrogen peroxide and superoxide when irradiated at 366 nm, but the rate of H2O2 production increases significantly in the presence of an FWA. When wet FWA-treated wool fabrics are irradiated with simulated sunlight, they produce significantly more H2O2 (by a factor of four) than peroxide-bleached wool. Photogeneration of H2O2 and O2− by electron transfer reactions from the excited state of the FWA, rather than energy transfer to 1O2, probably contribute significantly to the rapid photoyellowing of wet FWA-treated wool and silk fabrics which remains a serious commercial shortcoming of these fibres.